In the Back-End-of-Line (BEOL) processing of semiconductor devices, the remaining photoresist (PR) layer after plasma etch (post-etch photoresist or hardened photoresist) is traditionally removed using an oxygen containing plasma process, also known as ashing. Plasma processes and more particularly oxygen containing plasma process are known to induce damage to porous (low-κ) dielectric materials that are exposed during the photoresist removal, in such an extent that the performance of the semiconductor devices is deteriorated.
To minimize damage to the porous (low-κ) dielectrics, alternative wet chemistry methods mostly based on organic solvents are gaining a renewed interest. However, the presence of a “crust” generated by the etch plasma at the PR surface makes it impossible to completely remove the plasma etched photoresist by a pure organic solvent. The crust, most likely composed of cross-linked polymers, is not soluble in several types of organic solvents.
Porous dielectrics, also known as low-κ dielectrics are dielectric materials having a dielectric constant κ lower than the dielectric constant of silicon dioxide (κSiO2=3.9). Specifications on low-κ dielectric material loss and κ-value integrity (preserving the κ-value of the porous dielectric material upon photoresist removal) are difficult to meet by the purely wet-chemistry methods.
A challenge is to remove hardened photoresist without damaging (neither chemically, nor structurally) or etching the exposed low-κ materials. This challenge is particularly difficult to meet in single-wafer (SW) processing, required to remove hardened PR in shorter time intervals (e.g., about 1 min, while batch systems take from 10 min to 30 min). Increasing the concentrations of the chemicals to meet throughput requirements is not a valid solution when low-κ materials are exposed.
Wet strip chemistries (i.e. a mixture of chemical substances (or solution) in the liquid phase, comprising a solvent) are usually based on organic solvents, since hardened PR comprises polymers that are not soluble in water. However, more strict environmental regulations call for a reduction in the Environmental, Safety & Health (ESH) impact of solvents used in manufacturing. Beside ESH aspects, organic solvents usually bring higher costs from components and waste treatment.